If it's for small places that you can easily exit if you need to make a call, and signs are clearly posted indicating that jamming is taking place, I have no problem with it.
If it becomes so widespread that it becomes hard to make or receive calls in public during the day, I will definately reconsider that opinion. I carry one with my virtually every waking hour I'm not at home, and in fact it is my ONLY phone (I don't have a land line). Thus in a way I am reliant on it. I keep it on vibrate, and if I do answer it in a space, I will quietly and quickly tell the person I have to call them back/step out and do the same (or just hit "ignore" and pick up the voice mail later).
I think a lot of people just need to learn cell phone ettiquette, but there's no real way to enforce that, making these types of things necessary.
Now if you're a doctor on call or something, I can see the dillema. Too bad there isn't a way to just allow certain frequencies like that through (emergency frequencies?) to avoid that possible problem (if you're a doc on call, don't go into a building with jamming on.... an important reason to have signs clearly posted).
Cell phones HAVE saved lives in some cases, so I would be opposed to widespread jamming for fear of eliminating those situations (even if they are rare in public).